I live about an hour south of DC, but we go up there often and DH used to work up that way. I don't know your budget or tastes, but in DC there are several really nice places one of them being a Mandarin Oriental which is on
DVC's Concierge List, JW Marriot, The Mayflower, etc. When we go up there we stay in Arlington/Pentagon City at the Doubletree which is near the Metro and we have also stayed at The Ritz Carlton (they were having a great special going on!) and that's attached to Pentagon City Mall and we got free breakfast too, which was awesome! You can also walk directly into the mall from The Ritz and I believe the Metro runs right under it. There are several nice, safe hotels right in that area within walking distance to the Metro. IMO, I'd recommend staying at one of those and use the Metro.
Parking and Traffic are nightmares and I will never ride another cab there (or any other city). When using the Metro, pay attention to closing times. One time I parked at Arlington National Cemetary and didn't know they closed at 9pm, but the Metro closed at 9:30 or 10pm back then. My car got locked in the cemetary and we had to call the Park Police to get us out! Luckily, the officer grew up close to the town I grew up so he was nice to us.
A really nice dinner (or they do lunch or brunch cruises) is the Odyssey in DC. It's a dinner cruise with REALLY, REALLY good food and you get to see DC and Georgetown from the water (unless it's choppy, then you see DC and Alexandria, VA) check out the website. I don't know it off hand, but do a search for Odyssey dinner cruises. If your DH is into any Civil War stuff Fredericksburg/Spotsylvania area has many battlefields and tours of Historical Downtown Houses, etc. HTH